Honestly I'd have to agree with a Plat DLC coming. At least, I wish it happens. I personally hate the idea of DLC being over $10 (which it probably would be like SwSh) but a Platinum DLC would literally just fix the problems with this game, except for things in design that GF and ILCA just don't want to say "my bad for putting this in" and taking it out, like the shared EXP share.
I know that we'll get more patches and updates down the line, since we still need Home compatibility, the GWS, and more bug fixing. Mythical events are coded into the game, but aren't released. It'll probably be available as some crazy limited time event down the line.
Home compatibility will probably be there after you do something in the early game like beat the first gym(when you meet Bebe I guess?) or something way later that I can't think of. Having Home will change a lot though, because I know people will use it to get the national Dex early, if they let that be a thing. You can already get it early by trading a living Dex over with another switch, and I'm pretty sure it was intentional. You can do stuff like this in other games and every Pokemon, even from the nat dex, is set to the normal level ranges after beating a gym. Everything is just about the same, except for things that need the game completed (like going to the fight area).
Having Home compatibility will be really good for this game, but still makes fans like me upset because it's another workaround for having better content earlier(Rotom, Eevee, etc.).
I definitely think we'll get some more updates n stuff down the line, even with Legends Arceus coming out. The game's been selling, so they probably won't ignore it. I wish we get a lot more than what we have now though. I love Sinnoh and am enjoying my BDSP playthroughs, but it could be better.